The late 1980's witnessed a resurgence of both systemic infections and rheumatic fever. We demonstrated that a new clone of an old M serotype was associated with most cases of serious disease. This clone harbors a bacteriophage which encodes the SPEA toxin and many other genes which could contribute to its invasive phenotype. Preliminary experiments showed that this new invasive clone has enhanced capacity to penetrate human epithelial cells. The primary objective of this project is to understand the molecular basis for the propensity of highly virulent strains of group A streptococci to invade the blood stream and cause systemic disease, such as vascular occlusion and rheumatic fever. We will confirm that group A streptococci can penetrate and grow within cultured human lung cells and that the recently emerged invasive clone of M1 streptococci is better able to invade and multiply in human cells relative to less invasive strains. We will test whether bacteriophage DNA harbored by these invasive strains genetically encode for the invasive phenotype, and attempt to clone responsible genes and define their products. Resistance of streptococci to gentamicin and penicillin in association with cultured epithelial cells is the criterion used as a measure of the intracellular state.